zeronet.io at GitHub is created by The Pirate Bay founders, mainly by the Peter Sunde(ZeroNet username: nofish), after they reached a conclusion that it is not possible to run any central form of The Pirate Bay servers.
The details are described at the upstream documentation, which has a mirror zite(a "zite" stands for "ZeroNet site"), but the general idea is that there is a ZeroNet specific document format, each document, also called as a zite, is a folder with a "standardized" layout and some other requirements, and as people surf the ZeroNet, the documents/zites are downloaded by using the BitTorrent protocol and stored locally, at a local server. The locally stored copies are used for local viewing and for serving to others by using the BitTorrent protocol. The documents/zites are signed by using public key cryptography and they can be "edited" by signing and publishing a new version of the zite. Some of the documents are collectively editable and that functionality allows the creation of forums("ZeroTalk") and mail exchange portals("ZeroMail").
Each zite can declare some files to be optional. The optional files are downloaded only, when a user explicitly declares that it wants to download the optional files. The end user user can adjust the maximum locally allowed storage size of a particular zite.